RTEMS GPIO API interrupt handling

Sun Jul 19 13:24:38 UTC 2015

On July 19, 2015 5:24:25 AM CDT, "André Marques" <andre.lousa.marques at gmail.com> wrote:
>On 18-07-2015 20:04, Joel Sherrill wrote:
>> On July 18, 2015 1:23:13 PM CDT, "André
>Marques"<andre.lousa.marques at gmail.com>  wrote:
>>> Hello all,
>>>
>>> as previously pointed by Gedare, the interrupt handling code in the
>>> RTEMS GPIO API can benefit with the use of a rtems interrupt server.
>>> The
>>> current design and implementation details of the API can be seen in
>my
>>> GSOC blog
>>>
>(https://asuolgsoc2014.wordpress.com/2015/07/14/rtems-gpio-api-status/).
>>>
>>> To summarize the GPIO interrupt handling requirements it is
>important
>>> to
>>> note that a GPIO bank is composed of several pins all triggering a
>>> common interrupt vector, each pin being a separate interrupt source,
>>> which may in turn have one or more handlers of their own. This does
>not
>>>
>>> mean, however, that the pins interrupt handling is completely
>>> independent as although each pin has their own handler, we can not
>>> simply call all pin handlers on a GPIO bank every time a interrupt
>is
>>> sensed on the vector, as the pins themselves do not have interrupt
>>> state
>>> data.
>>>
>>> The active/pending interrupts on a GPIO bank are stored on a
>register,
>>> meaning that every time an interrupt is sensed on an interrupt
>vector
>>> the interrupt vector handler (which should be unique) will have to
>read
>>>
>>> the register to know which pins have pending interrupts, and then
>call
>>> the respective handler/handlers.
>>>
>>> Each pin may have more than one handler if more than one device is
>>> connected to the same pin, in which case each device will have a
>>> handler
>>> which should begin by probing their corresponding device to know if
>the
>>>
>>> culprit is their device, and if so handle it.
>>>
>>> The pin's handlers are stored on an internal (to the API) chain, and
>>> called sequentially as in this case each connected device will have
>to
>>> be probed to know the origin of the interrupt (shared interrupt
>handler
>>>
>>> behavior), and these handlers are dealt with by the API.
>>>
>>> Regarding the handling at the vector level, each bank currently has
>an
>>> unique rtems interrupt handler which probes the interrupt line on
>that
>>> bank, and calls (wakes) the corresponding pin handler task on any
>pin
>>> with a pending interrupt.
>>>
>>> This is the part that can be replaced with a rtems interrupt server,
>as
>>>
>>> each bank can have the "same" unique handler installed on the server
>>> through a call to rtems_interrupt_server_handler_install, which will
>>> put
>>> the vector/bank handler on a chain to be processed on the server
>task
>>> (which is waked every time an interrupt is sensed). The advantage
>>> relative to my current implementation is that this allows a single
>task
>>>
>>> to call every handler, instead of waking a task per pin which calls
>the
>>>
>>> corresponding pin handler(s) which is unnecessary overhead on a
>single
>>> core system (a SMP system could benefit of multiple tasks/servers to
>>> allow interrupt handling parallelism, although the current interrupt
>>> server implementation only allows a single server in the whole
>system.
>>> In this situation it would also be useful if the vector was
>re-enabled
>>> as soon as possible so any interrupts generated during the handling
>of
>>> a
>>> pin's interrupt can be quickly handled in parallel, instead of
>waiting
>> >from the previous interrupts to be processed - remember that in a
>GPIO
>>> vector each pin interrupt is independent - unless it is an interrupt
>on
>>>
>>> the same pin). The handler that will be called by the rtems
>interrupt
>>> server task will probe and clear the GPIO bank interrupt line, and
>call
>>>
>>> the necessary pin handlers before allowing the interrupt server to
>>> re-enable the vector.
>>>
>>> Apart from this, the API may also allow interrupts on a given
>>> vector/bank to be non-threaded (which is to say that they are
>handled
>>> on
>>> a regular interrupt handler, with the advantages/restrictions of an
>ISR
>>>
>>> environment).
>>>
>>> This is the current work plan regarding the GPIO interrupt handling
>in
>>> the RTEMS GPIO API, so if anyone has any issue with it do let me
>know.
>> For single pins that someone wants to map to an ISR, this sounds
>good. But it is not uncommon to have a set of pins that are a single
>logical source of interrupts.
>>
>> I have seen a case where multiple pins indicated the state of a 1024
>position encoder. And other uses of 2-4 pins for device state.
>>
>> How will these cases be dealt with?
>
>*If I understood correctly* the point is that a set of pins may perform
>
>the same action if any of them generates an interrupt, so in practice 
>they will share the same handler. The question would then be how to 
>synchronize the interrupt handling, such that only one handler instance
>
>is executing at a time. With the use of the interrupt server, since
>only 
>one task will call the handlers in sequence, if such a set of pins is 
>contained in a single vector/bank then the API already handles it 
>because even if two pins fire an interrupt the handlers will be called 
>one at a time, and the vector is disabled during this period.

Ok. So just install one handler for multiple pins in a bank. And check all pins when invoked?

>In fact, since the API requires a separate table for interrupt 
>configuration from the broader pin configuration table, it is possible 
>to define a single interrupt configuration and use that same table to 
>configure all the pins in this situation.

I don't know if one handler per bank would help directly. In the past, I used something like this and added a layer that checked for changes and fired change handlers for specific big sets which changed. 

For example, a 16 bit bank could be logically two 4-bit inputs, a single 2-bit input, and 6 single bit inputs. One interrupt handler could handle the HW interrupt and then invoke the appropriate change handlers.

This just required registering a mask and handler. The single ISR iterated over a table of that information.

>A problem may happen however if this set of pins span across more than 
>one vector/bank, as we might have two instances of the same handler in
>a 
>race condition. The solution may be to have a mutex per "logical 
>interrupt" as we might call this use case.

I wouldn't worry about this. I am sure some perverse HW designer has done it but it is easy enough to add application logic in the unlikely event one finds this.

Short version, I think this is rare enough not to worry about.

>The API can currently handle pin groupings/sets as single entities, and
>
>it uses a mutex to synchronize the group operations since it can span 
>multiple interrupt vectors, but considering that a group could also
>have 
>multiple logical sources of interrupts, it may not make much sense to 
>use a grouping to deal with this situation, specially because the pin 
>groupings intention is that someone will interact directly with the 
>group (reading/writing).
>
>A similar process can however be used.
>
>The idea would be to have an opaque type such as 
>rtems_gpio_logical_interrupt which would be defined as:
>
>struct rtems_gpio_logical_interrupt
>{
>   rtems_chain_node node;
>   rtems_id mutex;
>}
>
>Then the BSP/application could create a logical interrupt with
>something 
>like
>
>rtems_gpio_logical_interrupt
>*rtems_gpio_create_logical_interrupt(void);
>
>Which would return a pointer to a structure instance with a mutex.
>
>Then each interrupt configuration table could have a field for this 
>pointer, which when not NULL would require the mutex to be acquired 
>before calling the handler, hence synchronizing the access to the 
>handler to any pin sharing the same handler with some other pin(s).
>
>>> After this is done another round of code review may start.
>>>
>>> Thank you for your time,
>>> André Marques.
>>> _______________________________________________
>>> devel mailing list
>>> devel at rtems.org
>>> http://lists.rtems.org/mailman/listinfo/devel
>> --joel

--joel